Method and system for vibration avoidance for automated machinery

ABSTRACT

A method for vibration avoidance in automated machinery produces actuator space-time contours that meet design objectives of the machinery while suppressing energy content at frequencies in the space-time contour, by concatenating multiple space-time contour segments together in such a way as to be mostly free of energy at the frequencies of interest while meeting other specified design goals. The segments used to construct these frequency-optimized-contours are a series of concatenated polynomial segments, the independent variable t being time. These segments can define the variable to be controlled (e.g. speed or distance) versus time, or define one of the controlled variable&#39;s time-derivatives (e.g., the slope of the speed vs. time, etc.). When these frequency-optimized-contours are fed as a command to a machine controller through an actuator or actuators, the energy at the frequencies of interest is low enough to avoid deleterious vibration from occurring while still meeting the machine performance objectives.

This is a divisional patent application of co-pending U.S. applicationSer. No. 11/744,425, filed May 4, 2007, entitled “METHOD AND SYSTEM FORVIBRATION AVOIDANCE FOR AUTOMATED MACHINERY” The aforementionedapplication is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains generally to the field of methods for minimizingunwanted system dynamics. More particularly, the invention pertains to amethod and system for vibration avoidance for use with automatedmachinery.

2. Description of Related Art

Automated machines often have performance limitations caused by thevibration within the mechanisms and the stationary structure to whichthese mechanisms are attached within the machinery. Often, no matter howmuch energy is applied to a mechanism it is unable to perform theassigned task faster than a certain rate because vibrations interferewith the successful completion of the task. In these machines, althoughthe application of more energy typically allows the mechanism to operatefaster, the increased energy causes the amplitude of the vibrations toincrease. The increased vibrations interfere with the machine'soperation, especially the timely and precise stopping of an apparatus ata target point or value.

In the past, these problems have been dealt with using engineeringmethods, e.g., carefully designing the machine elements to not vibrateby using special materials, increasing the mass and/or stiffness of theframe and carefully designing the overall structure of the apparatus toavoid vibration, etc. These methods can only solve the problem to alimited degree, tend to increase the assembled cost of the machine, andrequire additional design effort and engineering cost to implement.Sometimes the successful application of these mechanical solutions isnot possible because of constraints imposed by the very task the machineis attempting to perform.

Every contour that machine elements are asked to follow as a function oftime is referred to herein as a space-time contour where “space” is thedependent physical variable to be controlled (i.e., space variable) as afunction of the independent variable time. Typical physical variables tobe controlled might be the height of a car, the pressure of a vessel,temperature of a slab, speed of a rotating disk, angle of an arm, etc.The physical variable to be controlled is “forced” by an actuator thatmight be a hydraulic piston, pump, burner, etc.

To understand the problem more fully, one has to be aware that everyspace-time contour contains energy at multiple frequencies, and one cantransform space-time contours into a function of energy versus frequencyand vice versa. Typical space-time contours that machine elements areasked to follow are similar to an impulse and contain a broad spectrumof frequencies much like the impulse from a drumstick striking a surfacehas a broad spectrum of frequencies. If the drumstick strikes a foam padon a board the result is a dull thud because the pad is similar to welldamped machine elements without resonance so no frequencies areamplified or sustained. If the drumstick strikes a gong or a tuning forkthe result is sustained vibration. Imagine then, an exaggerated examplewhere the machine elements in a machine have a gong they are trying tomove as a load, or alternately a tuning fork within a mechanicaltransmission. The goal then is to have the machine elements follow aspace-time contour, for example, to get the gong and/or tuning fork frompoint A to point B while keeping the gong and/or tuning fork fromvibrating at their natural frequencies (exciting frequencies). Simplyput, this can be accomplished by removing the energy from the mechanicalactuator that moves from point A to point B at the exciting frequenciesthat would excite the gong and/or the tuning fork.

In the past, control methods have been employed to remove the energy atthe exciting frequencies so as to limit the vibration to help free themachine designer from vibration constraints. These control methods fallinto two different classes.

The first class of control method that has been employed is to removethe frequency or frequencies that cause problematic vibration (excitingfrequencies) directly from the energy source driving the mechanism(typically electrical current or hydraulic flow) through an actuator.(See, FIG. 1). This is accomplished by placing a frequency selectivefilter in the path of the energy source control. This method reduces theexciting frequencies entering the machine through the actuator. Feedbackis usually employed from sensor(s) on a machine element(s) to modulatethis power source through the filter to force the mechanism to followits desired space-time contour. This control method has limitations inthat the space-time contour command itself still likely has energy atfrequencies that will cause vibration in the machine. By removing theexciting frequencies from the energy source driving the machineelements, errors are induced limiting the faithful following of thespace-time contour, and often, these errors are unacceptable to themachine operation. In many instances, if the filtering of the energysource is not sufficient and/or robust, the feedback control, attemptingto follow the desired space-time contour will force energy back into thesystem at the exciting frequencies, limiting the usefulness of thiscontrol solution.

The second class of control method that has been employed is to reducethe level of exciting frequencies that cause vibration from thespace-time contour that the machine elements are to follow. (See, FIG.2). This is accomplished by taking a space-time contour which has beendesigned to meet the application objectives and routing it through afrequency elimination filter that selectively removes most of the energyat the exciting frequencies to produce a “non-exciting” contour. This“non-exciting” contour is then delivered as the command to the controlsystem that modulates the actuator driving machine elements in amachine. The contours for one or more apparatus within the machine maybe filtered in this way to reduce vibrations. This method has proven tobe effective to various degrees in reducing machine vibration, dependentupon the level of suppression provided by the filter and the robustnessof the filter's effectiveness with changes in the machine elements. Theunfortunate side effect of this technique is distortion of the“non-exciting” contour from the original desired space-time contour. Infact, specific versions of this method are often described as a methodto “shape” the space-time contour. These specific methods to shape acontour are, in fact, a specialized frequency selective filter that hasbeen designed by a specific time-domain technique, see U.S. Pat. No.4,916,635. Space-time contour filtering produces, in any case, a contourthat is distorted from the original designed space-time contour suchthat it usually degrades the performance of the machine, often causingit to not meet the design objectives. In the case of a space-timecontour that is intended to move a mechanical load from one point toanother and then stop, this method always increases the time betweenthese points because delay, inherent in the filter operation,necessarily lengthens the “non-exciting” contour. This has the likelyeffect of slowing the overall machine, reducing its operatingthroughput.

Thus, there is a need in the industry for an improvement over thesetechniques to allow more freedom from constraints in the design ofmechanisms within machines and/or to increase the throughput fromexisting mechanism designs without suffering the deleterious effects ofinduced vibration.

SUMMARY OF THE INVENTION

The new method described herein represents an advance over the prior artin that it produces space-time contours that meet the original designobjectives while suppressing the energy content at the excitingfrequencies in the space-time contour, said resulting space-time contourdefined as a frequency-optimized-contour. This method also takes intoaccount physical limitations within the machine system as a constraintsuch that the frequency-optimized-contour keeps one physical variable(speed, distance, force) within a bound, said bound being describedherein as a limitation of a physical variable.

We have developed a machine control system that produces thesefrequency-optimized-contours when given the goals the space-timecontours must attain (typically points in space and time) andconstraints on the frequency content of the resulting space-timecontour. Limits can be imposed based not only on the energy level at theexciting frequencies but the energy limit can be extended to a bandabout these frequencies. Using frequency bands as limits makes thetechnique robust should the exciting frequencies shift in the machinedue to manufacturing tolerance, wear, aging, drift, etc. of the machineelements. This band (or these bands) of frequencies are defined hereinas frequencies of interest which, along with their suppression criteriaare specified by frequency-elimination criteria. No expressions thatquantify the dynamics of the machine elements are required for thisinvention to operate properly, freeing the designer from tediousanalysis.

Frequency-optimized-contours are produced, without any post filtering orshaping, by concatenating multiple space-time contour segments togetherin such a way as to be mostly free of energy at the frequencies ofinterest while meeting the other design goals specified for thespace-time contour. The segments used to construct thesefrequency-optimized-contours are a series of concatenated polynomialsegments as further discussed below. When thesefrequency-optimized-contours are fed as a command to machine controllerthrough an actuator or actuators, the energy at the frequencies ofinterest is low enough to avoid deleterious vibration from occurringwhile still meeting the machine performance objectives.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic diagram illustrating a first prior art typeof control method employed to remove exciting frequencies.

FIG. 2 provides a schematic diagram illustrating a second prior art typeof control method employed to remove exciting frequencies.

FIG. 3 provides a schematic diagram illustrating a frequency optimizedcontour.

FIG. 4 provides a schematic diagram illustrating a preferred embodimentof a frequency optimized contour engine.

FIG. 5 provides a schematic diagram illustrating the use of a databaseof frequency optimized contours for a machine to be controlled for alltypes of contours that are expected to be required by the machinecontroller.

FIG. 6 provides a flowchart of a first embodiment of the method of theinvention.

FIG. 7 provides a detail flowchart of an optimization step which can beused in the embodiment of the method of the invention of FIG. 6.

FIG. 8 provides a flowchart of a second embodiment of the invention,using a database of contours.

DETAILED DESCRIPTION OF THE INVENTION

As previously noted, frequency-optimized-contours are produced in ourinvention, without any post filtering or shaping, by concatenatingmultiple space-time contour segments together in such a way as to bemostly free of energy at the frequencies of interest while meeting theother design goals specified for the space-time contour. The segmentsused to construct these frequency-optimized-contours are a series ofconcatenated polynomial segments where each polynomial segment can bedefined by the equation: x=k₀+k₁t+k₂t²+ . . . +k_(n)t^(n), etc. orequivalent algebraic forms, the independent variable t being time. Thesesegments can define the variable to be controlled (e.g. speed ordistance) versus time, or define one of the controlled variable'stime-derivatives (e.g., the slope of the speed vs. time, etc.).

As an example, if the frequency-optimized-contour to be designed was tobe a distance vs. time contour, the concatenated segments used toconstruct the contour could be linear ramp segments in speed vs. time(s=k₀+k₁t) where s, speed, is the time-derivative of distance. Examplesof higher order time-derivative segments used to build a distance vs.time contour would be segments in acceleration vs. time, jerk(derivative of acceleration vs. time), etc.

When these frequency-optimized-contours are fed as a command to machinecontroller through an actuator or actuators, the energy at thefrequencies of interest is low enough to avoid deleterious vibrationfrom occurring while still meeting the machine performance objectives.

There are four primary methods for producing such afrequency-optimized-contour:

A. By using an optimization engine that builds afrequency-optimized-contour from polynomial segments in atime-derivative of the physical variable to be controlled, concatenatedin time, and then time-integrating the segments one or more times, giventhe frequency-elimination criteria and space-time contour criteria ondemand, when it is required for machine operation.

B. By using an optimization engine that builds afrequency-optimized-contour from polynomial segments of the physicalvariable to be controlled, concatenated in time, given thefrequency-elimination criteria and space-time contour criteria ondemand, when it is required for machine operation.

C. By a space-time contour culling method that creates afrequency-optimized-contour by selecting a frequency-optimized-contourfrom a previously constructed database and scaling it to meet thecurrent demand from the machine control system. The Optimized ContourDatabase having been previously populated using one or more of themethods in A or B above with frequency-optimized-contours that are asuitable root set for machine operation.

D. By a space-time contour culling method that creates afrequency-optimized-contour by selecting a time-derivativefrequency-optimized-contour from a previously constructed database,scaling and integrating it one or more times to create afrequency-optimized contour that meets the current demand from themachine control system. Where a time-derivativefrequency-optimized-contour is a space-time contour where the dependentspace variable is a time-derivative of the physical variable to becontrolled, typically produced by method A above, beforetime-integration.

Methods C or D are typically employed when the delay in calculating afrequency-optimized-contour on-demand is too long for proper machineoperation.

To understand how the preferred embodiment of afrequency-optimized-contour engine 9 operates, refer to FIG. 4. It canbe seen that the process begins when a contour demand with space-timeconstraints 10 is delivered to the system. Using the actuator'scapability as a constraint 11 (e.g., the maximum force vs. speed curvefor the mechanical actuator 24), the prototype segmented contour builder13 creates an arbitrary segmented space-time contour that meets thespace-time constraints. This prototype contour 12 is processed by aspace-time to frequency transform engine 14 to produce the prototypecontour's spectral content 15 (the energy of the frequencies that arepresent in the prototype contour). The contour rating machine 16 thenrates the prototype contour 12 against the contour rating criteria 18and the prototype contour's spectral content 15 against thefrequency-elimination criteria 17, which is the maximum level of thefrequencies of interest. The contour's specification is then stored in arated contour database 19 along with a quality rating that describes howclose the contour meets the criteria 17 and 18.

The prototype segmented contour builder 13 continues to fill the ratedcontour database 19 with contour specifications using the above methoduntil a sufficiently diverse initial prototype set of contours isproduced (seed contours). The level of diversity (number of seedcontours) that is sufficient is dependent on the method used within theperturbation engine 20 and the method used within the prototypesegmented contour builder 13 and may be as small as one seed contour.

Once a sufficient diversity exists within 19 as detected by theperturbation engine 20, it takes one or more of the contourspecifications therein, modifies and/or mixes it (them) to produce aspecification for a new contour, which is fed to 13. At this point, theprototype segmented contour builder 13 ceases to build the seed contoursand instead builds the contours requested by the perturbation engine 20which are also analyzed by 14, rated by 16 and their specification andquality rating is stored in 19. This process continues with theperturbation engine producing prototype sets of contour specificationsthat are likely to contain improved contours as rated by 16 until asuitable contour is produced that meets the criteria 17 and 18 or untila time limit is reached. Once this occurs, a signal is sent to theperturbation engine 20 to stop producing contour specifications. It maystop immediately or it may continue to complete a set of contourspecifications it is working on.

Once the perturbation engine 20 has ceased work, the contour ratingmachine 16 signals the segmented contour generator 22 that afrequency-optimized-contour 27 has been constructed and passes it theindex of this contour's specification within the rated contour database19. The segmented contour generator 22 processes the indexedspecification into a full space-time contour, which is used to commandthe actuator control 23. The segmented contour generator 22 may performtime-integration on the series of concatenated segments, one or moretimes, depending on the mode of the frequency-optimized-contour engine9. Time-integration, when used, produces the mathematical integral ofthe series of concatenated segments with respect to time in either acontinuous or discrete time mode. As used herein, the termstime-integrated and time-integrating are verbs that are defined asperforming the act of time-integration.

The actuator control 23 modulates a power source in order have theactuator 24 follow the commanded frequency-optimized-contour in afaithful manner. The actuator control 23 may or may not use a feedbackloop 26 to accomplish the faithful control of the actuator 24. Themachine elements 25, having been forced by thefrequency-optimized-contour will vibrate at a minimum level because thevibration inducing exciting frequencies have been largely removed fromthe actuator output as specified by the frequency-elimination criteria17.

It should be noted that the resulting frequency-optimized-contourproduced by this method is also optimized to meet energy and space-timecriteria as defined by 18. A typical space-time criteria defined by 18would be to minimize the total elapsed time of thefrequency-optimized-contour, and 18 might also include other space-timeconstraints, energy usage constraints, actuator loading constraints andother constraints and/or performance metrics.

If the demand for a frequency-optimized-contour must be executed fasterthan the frequency-optimized-contour engine can create one, an alternatesupplementary machine may be used to produce similar results that cancomplete the task in substantially shorter time. This is accomplished bycreating a database of frequency-optimized-contours for the machine tobe controlled for all types of contours that are expected to be requiredby the machine controller. Alternately, this database can be constructedto contain a set of frequency-optimized-contours sufficiently rich suchthat for every contour demand that is expected to be requested by themachine controller one within the database can be selected and scaled tomeet the demand requirements.

Once the database of frequency-optimized-contours is fully populated(typically done before the machine is running), then a culling apparatusis used to produce the frequency-optimized-contours quickly whendemanded to do so by the machine controller. The culling machine, uponbeing requested for a frequency-optimized-contour by the machinecontroller, searches the database for the closest matchingfrequency-optimized-contour that can be scaled to meet the machinecontroller's request. To scale this contour, a scaling machine, withinthe culling apparatus, multiplies the dependent variable (e.g., speed,acceleration, jerk, pressure, etc.) of the contour by a constant equallyat all points in time to stretch or shrink the resulting contour to meetthe machine controller's request. Alternately, the scaling machine canproportionally shrink or stretch the frequency-optimized-contour in time(making the elapsed time of the contour longer or shorter). The latterhas the effect of scaling the frequencies that are suppressed.

Detailed Explanation of the Method

Referring to the flowchart in FIG. 6, an embodiment of the method ofproducing a space-time contour of the invention comprises:

-   Step 100—starting with a series of concatenated polynomial segments    that are functions of time where their dependent variable is a    time-derivative of at least the first order of a physical variable    to be controlled, each polynomial segment comprising a duration and    at least one coefficient;-   Step 110—optimizing the durations and coefficients of the series of    concatenated polynomial segments, such that the following    constraints are met: energy that would be introduced into at least    one machine element by an actuator following a space-time contour    created by time-integrating the series of concatenated polynomial    segments, is suppressed for at least one frequency, and a space-time    contour created by time-integrating the series of concatenated    polynomial segments, is constrained within at least one limitation    of the physical variable.

Optionally, the optimization of step 110 could also include otherconstraints, such as a space-time contour created by time-integratingthe series of concatenated polynomial segments will be at one or morepoints in a controlled variable space at selected points in time, or thesum of durations of the concatenated segments of the series ofconcatenated polynomial segments is minimized, or energy dissipated bythe at least one actuator following a space-time contour created bytime-integrating the series of concatenated polynomial segments isminimized, or other constraints as might be required by the application.During optimization, segments could be added or deleted from the series.

-   Step 120—time-integrating, at least once, the series of concatenated    polynomial segments to form a space-time contour in the physical    variable to be controlled;-   Step 130—commanding an actuator to follow the space-time contour.

It will be recognized that this method can also be performed if thedependent variable of each segment is a physical variable to becontrolled, and each polynomial segment comprises a duration and acoefficient, forming a space-time contour in the physical variable. Inthat case, the integration step 120 would not be required.

Referring to FIG. 7, the optimization of step 110 can utilize a searchmethod which may comprise the steps of:

-   Step 111—using at least one capability of the actuator as a    constraint, forming a prototype set from at least one series of    concatenated polynomial segments;-   Step 112—calculating for each series of concatenated polynomial    segments in the prototype set, the energy at a set of frequencies    that would be introduced into at least one machine element by an    actuator following a space-time contour created by time-integrating    each series of concatenated polynomial segments. If desired, a    time-frequency transform process can be used to calculate the    energy.-   Step 113—assigning a quality rating to each series of concatenated    polynomial segments within the prototype set using the energy    calculated in step b, the quality rating being related to the    suppression of energy at the at least one frequency.

This quality rating could optionally be related to the sum of durationsof the concatenated segments of the series of concatenated polynomialsegments, or a capability of an actuator to follow the space-timecontour, or a total energy dissipated by an actuator when forcing amachine element to follow the space-time contour.

-   Step 114—perturbing at least one of the series of concatenated    polynomial segments within the prototype set to produce a new    prototype set; and-   Step 115—iteratively repeating steps 112, 113, and 114 until the    best quality rating approaches a determined value, then-   Step 116—selecting the series of concatenated polynomial segments    within the prototype set with the best quality rating.

Referring to FIG. 8, another embodiment of the invention provides amethod for providing a frequency-optimized contour to a machine controlsystem requiring a contour comprising a physical variable to becontrolled which changes with time as an independent variable duringmachine run time, comprising:

-   Step 150—creating a database of frequency-optimized contours by the    steps of:-   Step 152—finding a plurality of constraints for a set of contours    such that, for each contour that is required during machine run    time, there is at least one contour within the set that can be    scaled to fit the requirement;-   Step 154—producing a set of frequency-optimized-contours that meets    said constraints; and-   Step 156—storing said set of frequency-optimized-contours in a    frequency-optimized-contour database; and-   Step 160—determining an application requirement for a use of the    machine control system;-   Step 170—finding a frequency-optimized contour in the database that    is suitable for amplitude scaling to meet the application    requirement; and-   Step 180—scaling the frequency-optimized contour found in step c by    multiplying a variable by a constant.

Optionally, the constant could be the number “one”, in which case thevariable remains the same and the scaled contour is the same as theunscaled contour. The variable could be the physical variable, and themultiplication by a constant is performed equally at all points in timeto produce the scaled contour, or the variable could be the timevariable, and the multiplication by a constant is performed equallythroughout the contour to produce the scaled contour. The variablecould, if desired, be time-integrated.

-   Step 190—providing the scaled frequency optimized contour to the    machine control system.

However, reference herein to details of the illustrated embodiments isnot intended to limit the scope of the claims, which themselves recitethose features regarded as essential to the invention. Accordingly, itis to be understood that the embodiments of the invention hereindescribed are merely illustrative of the application of the principlesof the invention.

1. A method for providing a frequency-optimized contour to a machinecontrol system requiring a contour comprising a physical variable to becontrolled which changes with time as an independent variable duringmachine run time, comprising: a) creating a database offrequency-optimized contours by the steps of: i) finding a plurality ofconstraints for a set of contours such that, for each contour that isrequired during machine run time, there is at least one contour withinthe set that can be scaled to fit the requirement; ii) producing a setof frequency-optimized-contours that meets said constraints; and iii)storing said set of frequency-optimized-contours in afrequency-optimized-contour database; and b) determining an applicationrequirement for a use of the machine control system; c) finding afrequency-optimized contour in the database that is suitable for scalingto meet the application requirement; and d) scaling thefrequency-optimized contour found in step c by multiplying a variable bya constant; e) providing the scaled frequency optimized contour to themachine control system.
 2. The method of claim 1, where the constant instep d is fixed as the number one.
 3. The method of claim 1, in whichthe variable in step d) is the time variable, and the multiplication bya constant is performed equally throughout the contour to produce thescaled contour.
 4. The method of claim 1, in which the variable in stepd) is the physical variable, and the multiplication by a constant isperformed equally at all points in time to produce the scaled contour.5. A method for providing frequency-optimized contours to a machinecontrol system requiring a contour comprising a physical variable to becontrolled which changes with time as an independent variable duringmachine run time, comprising: a) creating a database offrequency-optimized contours by the steps of: i) finding a plurality ofconstraints for a set of contours such that, for each contour that isrequired during machine run time, there is at least one contour withinthe set that can be scaled to fit the requirement; ii) producing a setof frequency-optimized-contours that meets said constraints; and iii)storing a time-derivative of each of the said set offrequency-optimized-contours in a frequency-optimized-contour database;and b) determining an application requirement for a use of the machinecontrol system; c) finding a frequency-optimized contour in the databasethat is suitable for amplitude scaling to meet the applicationrequirement; and d) producing a scaled frequency-optimized contour foundin step c) by multiplying a variable by a constant; and time-integratingthe variable at least once; e) providing the scaled frequency optimizedcontour to the machine control system.
 6. The method of claim 5, wherethe constant in step d) is fixed as the number one.
 7. The method ofclaim 5, in which the variable in step d) is the time variable, and themultiplication by a constant is performed equally throughout the contourto produce the scaled contour.
 8. The method of claim 5, in which thevariable in step d) is a time-derivative of at least first order of thephysical variable, and the multiplication by a constant is performedequally at all points in time to produce the scaled contour.